studies of catalyst deactivation in methanol conversion with high, medium and small pore...

Applied Catalysis A: General 192 (2000) 85–96

Studies of catalyst deactivation in methanol conversion with high,medium and small pore silicoaluminophosphates

J.M. Campelo∗, F. Lafont, J.M. Marinas, M. OjedaOrganic Chemistry Department, Faculty of Sciences, University of Cordoba. Av. San Alberto Magno s/n, E-14004 Cordoba, Spain

Received 13 May 1999; received in revised form 26 July 1999; accepted 27 July 1999

Abstract

The activity, stability and selectivity of a series of silicoaluminophosphate (SAPO) molecular sieves have been investigatedin a flow reactor, operating at 400◦C. SAPO-5 and SAPO-34 catalysts exhibit higher activity than SAPO-11, SAPO-31and SAPO-41, according to their acidic properties determined using ammonia and pyridine as probe molecules. The mainproducts obtained with the most active solids are light olefins (SAPO-34) and C4–C7 hydrocarbons (SAPO-5), but their activitydecreases strongly with time due to coke formation. The other catalysts give high selectivities to dimethyl ether (DME). Theselectivity patterns found are well interpreted in terms of a series of reaction pathways incorporating both shape-selectivityfactors and distribution of acid strength. The nature of the coke formed has been investigated by MS, FT-IR and MAS NMRand it indicates the formation of polyunsaturated long chain hydrocarbons. ©2000 Elsevier Science B.V. All rights reserved.

Keywords:Catalyst deactivation; Methanol conversion; Silicoaluminophosphates

1. Introduction

Since the first report by Chang and Silvestriin 1977 [1], a considerable research effort hasbeen directed towards research on different as-pects of methanol-to-gasoline conversion usingzeolite catalysts [2,3]. Methanol is one of thelargest chemical commodities that can be pro-duced from natural gas, biomass, or from coalvia synthesis gas and that can be used as a fuel,fuel precursor, and building block for chemi-cals. There are already some commercial tech-nologies for the production of hydrocarbons from

∗ Corresponding author. Tel.:+34-57-218623;fax: +34-57-218606.E-mail address:[email protected] (J.M. Campelo).

methanol (for instance, the MTG process of Mobil[4]).

Molecular shape-selective acid-catalyzed synthesisof ethene and propene from methanol proceeds oversome types of molecular sieves having structures withsmall pores, like erionite, zeolite T, chabazite andZK-5 [5]. Unfortunately, the coking is rapid on thesecatalysts. Some modified forms of ZSM-5 with basicadditives to reduce acid strength have been proved tohave enhanced selectivity towards lower alkenes andlow coking rates, but their activity is reduced greatly[6]. The coking reactions are a complex function ofBrönsted acid center strength distribution and con-centration in a given topology since they determinethe possible forms of molecular shape selectivity con-straints imposed on the reactants, products, transitionstates and molecular diffusion. The right combinationof all these factors gives the key for the design of

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86 J.M. Campelo et al. / Applied Catalysis A: General 192 (2000) 85–96

active, selective, and stable catalysts for the synthesisof lower olefins from methanol.

The most straightforward way to change the acidstrength and concentration of acid centers is by theisomorphous substitution of elements having differentatomic electronegativities, concentrations and distri-butions in the framework of the molecular sieve.

Aluminophosphate-based molecular sieves haveopened research in this direction [7–9]. These solidsform open crystalline structures with channels andcavities of molecular dimensions. Aluminophosphateframeworks are electroneutral, but they can be mod-ified by the isomorphous substitution of either alu-minum or phosphorus thus giving negatively chargedframeworks called silicoaluminophosphates (SAPOs)(if silicon is incorporated into the aluminophosphateframework).

This study aims at examining factors controlling theactivity and selectivity of methanol conversion withSAPOs and completing our previous studies with thiskind of acid solids [10–15].

2. Experimental

2.1. Catalyst preparation

All the microporous solids were synthesized in ourlaboratory according to previous studies [10,13] wherethey were crystallized hydrothermally from the gelmixture containing inorganic oxides (Al, P and Si) andorganic templates. The gel composition and synthesisconditions are presented in Table 1. In the preparationof these molecular sieves, the formation of a uniformgel mixture is one important task. The gel is preparedby slow addition of the aluminum source (with con-tinuous stirring) to the partly dilutedortho-phosphoricacid solution and the mixture is stirred until a uniformgel is produced. Fumed silica is then added with con-tinuous stirring to the aluminophosphate gel mixture,and after complete addition, the silica aluminophos-phate gel is stirred for 1 h. The final gel mixture is pre-pared by slow addition of the organic template over aperiod of 10–15 min. The gel mixture was crystallizedhydrothermally in a 250 cm3 Teflon coated stainlesssteel autoclave. The solid reaction is washed with hotdistilled water, dried at 120◦C for 24 h and air calcinedat 600◦C for 6 h.

2.2. Characterization of catalysts

The chemical compositions of solids (Al, Si and P)were determined by EPMA using a Kervex analysermod. 8000, and SEM micrographs were obtained ina mod. SS40 ISI apparatus. X-ray diffraction patternswere recorded on a Siemens D-500 diffractometer(35 KV, 20 mA) using Ni-filtered Cu Ka radiation andan angle scanning speed of 2◦ min−1 between 5 and60◦. N2 isotherms were obtained at−190◦C with aMicromeritics ASAP-2000.27Al, 31P and29Si MASNMR spectra were obtained with a ACP-400 Brücker(external magnetic field 9.4 T). Diffuse-reflectanceFTIR spectra were recorded on an FTIR instrument(Bomen MB-100) equipped with an environmentalchamber placed in a diffuse-reflectance cell.

2.3. Surface acidity measurements

The surface acidity was determined by the follow-ing:1. Gas-phase adsorption pyridine using a pulse chro-

matographic technique in which portions of 20 mgof catalyst are placed into the injection port of aHP 5890 gas chromatograph, and saturated withknown amounts of pyridine [16].

2. Temperature-programmed desorption (TPD) ofammonia based on high-resolution mass spec-trometry detection. In this case, saturated ammo-nia catalysts in helium stream were ramped from75 to 450◦C at 8◦ min−1, and connected on-line toa VG AutoSpec high resolution mass spectrome-ter (Micromass). TPD profiles were obtained bymonitoring m/z= 17.026 from ammonia (exactmass from molecular mass peak of ammonia).

3. Diffuse reflectance infrared (DRIFT) spectra ofhydroxyl groups and of adsorbed probe molecule,in our case pyridine. These experiments werecarried out on a Bomen MB-100 IR instrumentequipped with a temperature controlled environ-ment chamber in which solids are maintainedfrom room temperature to 300◦C [13].

2.4. Methanol conversion

Catalytic activity measurements were performedusing a conventional continuous flow reactor at

J.M. Campelo et al. / Applied Catalysis A: General 192 (2000) 85–96 87

Table 1Preparation of catalysts

Catalyst Al P Si Template tr (h)a T (◦C)b

SAPO-5 Boehc H3PO4d SiO2 (s)e TPAf 24 200

SAPO-11 Boeh H3PO4 SiO2 (f)g DPAh 24 200SAPO-31 Boeh H3PO4 SiO2 (s) DPA 24 200SAPO-34 Boeh H3PO4 SiO2 (f) TEAOHi + DPA 24 200SAPO-41 Boeh H3PO4 SiO2 (f) DEAj 48 190

a Time of crystallization.b Temperature of crystallization.c Pseudoboehmite (AlOOH).d Phosphoric acid, 85 wt.%.f Tri-n-propylamine.e Silica sol, 30 wt.%.g Fumed silica.h Di-n-propylamine.i Tetraethylammonium hydroxide.j Diethylamine.

atmospheric pressure. Methanol (25ml min−1) in he-lium gas (30 ml min−1) was fed over SAPO catalysts(20 mg), at 400◦C. Products eluted from the reac-tor were analyzed directly by GC-FID (previouslycharacterized by GC-MS on a VG AutoSpec massspectrometer) via a thermostated six-way valve. A100 m Petrocol DH fused silica capillary column wasused to separate the reaction products.

2.5. Coke characterization

Coke deposited during methanol-to-hydrocarbon re-actions was characterized by FT-IR,13C and1H MASNMR and direct insertion solid probe-high resolu-tion mass spectrometry. A VG AutoSpec EBE highresolution mass spectrometer (Micromass) was usedfor mass spectrometry experiments. Ionization wascarried out at 70 eV with the source temperature setat 150◦C. 0.2 mg portions of each deactivated cata-lyst were placed in a deep quartz sample capillarytube, which was inserted into the direct insertion solidprobe of the mass spectrometer. A glass wool plug wasplaced at the mouth of the capillary to stop the pos-sible flow of the sample when exposed to the sourcevacuum. Once the sample had been pumped for 2 minto 10−2 mbar, it was inserted into the source housingof the mass spectrometer (10−6 mbar) for acquisition.Mass spectral acquisition was carried out fromm/z 15to 600 scanning at 1 s per scan, while the probe wasramped (150◦C, 0.1 min, then 2◦C s−1 to 600◦C). Ac-

curate mass measurements were obtained by scanningat 10.000 resolution.

13C and1H MAS NMR spectra of the deactivatedcatalysts were obtained with a Brücker ACP-400.Diffuse-reflectance FTIR spectra were recorded on anFTIR instrument (Bomen MB-100) equipped with anenvironmental chamber placed in a diffuse-reflectancecell; portions of 15 wt.% deactivated catalysts (di-luted with KBr) were placed in this cell and heated at150◦C for 20 min previous to the aquisition of data.As reference material, we employed the pure dilutedcatalyst (without coke). In all cases, IR spectra wererecorded at 150◦C.

3. Results and discussion

3.1. Catalyst characterization

The XRD data of the samples studied showed thereflections obtained from each catalyst, which corre-spond to the patterns found in the literature for eachof them [7,8], and the interplanar spacing (d-values)calculated from the reflections were also in very goodcorrespondence with literature data. These patterns re-veal the highly crystalline nature of the solids as wellas an absence of any other crystalline phase in eachof these materials. These results agree with scanningelectron micrographs in Fig. 1, in which we can ob-serve agglomerates of a great number of small crystals


Fig. 1. SEM photographs of SAPO-5 (a), SAPO-11 (b), SAPO-31 (c) and SAPO-34 (d) catalysts.

with spheric shapes and sizes between 3 and 15mm,and with a very low concentration of amorphousmaterial.

The chemical compositions (EPMA) of the molec-ular sieves used in the present study are given inTable 2. The silicon atom can be substituted into thealuminophosphate framework via [1] silicon substitu-tion for phosphorus, [2] simultaneous substitution oftwo silicons for one aluminum and one phosphorus,or [3] silicon substitution for aluminum [17,18]. Thefirst and the second mechanism would be expectedto occur in the SAPOs of this study. From chemicalcomposition, we can conclude that the substitution ofsilicon for phosphorus predominates in all cases.

Textural properties indicate the high degree of mi-croporosity in these solids, especially SAPO-34 whosemicropore area is very close to its BET area.

29Si MAS NMR spectra (results not shown) indicatethe existence of several lines, one characteristic of allof them between−91 and−96 ppm corresponding toSi(4Al) units [19–21] resulting from the substitution ofsilicon for phosphorus in the framework (Mechanism1), and in the case of SAPO-34, it is the single line thatthis solid exhibits (and which explains the frameworkcomposition of this catalyst) [21–23]. The incorpo-ration of isolated Si atoms generates SiOHAl groupswith strong acid properties. The presence of otherbands at lower ppm (in the range of−95–−109 ppm)


Table 2Chemical composition and textural properties of calcined samples

Sample Compositiona Smp (m2 g−1) SBET (m2 g−1)

SAPO-5 Si0.09Al0.48P0.43 128 183SAPO-11 Si0.03Al0.52P0.45 65 110SAPO-31 Si0.04Al0.52P0.44 63 115SAPO-41 Si0.03Al0.52P0.45 83 91SAPO-34 Si0.15Al0.50P0.35 117 119

a Expressed as (SixAlyPz)O2.

in the29Si MAS NMR spectra indicates the existenceof multiple silicon environments (rich silicon envi-ronments) in which there are Si–O–Si linkages, espe-cially with SAPO-5, which have a high silicon con-tent and indicate the co-existence of other mechanismsof silicon incorporation (two silicon atoms for one ofaluminum and another of phosphorus, Mechanism 2)[24].

27Al MAS NMR spectra of all catalysts (results notshown) contain a single peak centered at ca. 36 ppm,typical of tetrahedral aluminum sharing oxygens withphosphorus tetrahedra (and some of silica) [25–27].31P MAS NMR spectra show, in all cases, a single res-onance line at ca.−30 ppm corresponding to P-atomsin tetrahedral coordination with P–O–Al bonds (i.e.P(OAl)4 environments).

3.2. Acidic properties

The TPD-MS profiles of ammonia desorption onSAPO solids are shown in Fig. 2. There are twomaximum peaks around 200 and 350◦C, correspond-ing to adsorption on weak and strong acid sites,respectively. In the case of SAPO-5, these two peaksappear at about 225 and 400◦C, which indicates thepresence of stronger acid sites. The total concentra-tion of acid sites in the samples are listed in Table3. The overall amount of ammonia desorbed (Ta-ble 3) falls in an interval between 0.5 mmol g−1 forSAPO-11, SAPO-31 and SAPO-41, and 0.9 mmol g−1

for SAPO-34. The high temperature peak assignedto the strong acid sites increases in the followingsequence: SAPO-11≈ SAPO-31≈ SAPO-41< SA-PO-5< SAPO-34. From TPD-MS data, we can seethat there are no strong acid sites on SAPO-11,SAPO-31 and SAPO-41 compared to SAPO-5, andSAPO-34.

Fig. 2. MS profile of m/z= 17.026 from TPD of ammonia onSAPO-41 (A), SAPO-31 (B), SAPO-11 (C), SAPO-34 (D) andSAPO-5 (E).

Ammonia desorption at a high temperature is usu-ally attributed to the strong acid sites (bridging SiO-HAl groups) generated during the incorporation of sil-icon into the framework of the SAPO molecular sieve[28,29]. Weak acid sites are most probably Brönstedcenters originating from POH groups, which can be as-cribed to phosphorous atoms that are not fully linked toAlO4 tetrahedra [30] and SiOH groups. The presenceof these POH groups indicates imperfections withinthe structure of the framework.

By the gas chromatography pulse technique, the dis-tribution of acid sites has also been calculated, em-ploying pyridine as probe molecule. Table 3 gives theacidity (asmmol of pyridine adsorbed per g of cat-alyst, at 300 and 400◦C). In this case, we can also


Table 3Surface acidic properties of catalysts

Catalyst TPD NH3 (mmol g−1) Pyridine, 300◦C (mmol g−1) Pyridine, 400◦C (mmol g−1)

SAPO-5 686 259 210SAPO-34 870 41 29SAPO-11 503 107 83SAPO-31 496 60 28SAPO-41 525 109 60

observe that SAPO-5 is the catalyst with the highestnumber of strong acid sites, and SAPO-34 shows avery low adsorption value. It is noteworthy that pyri-dine adsorption on SAPO-34 was not even attained at300◦C, which is attributed to steric limitation on dif-fusion of this base into 4.4 Å diameter channels. Thissituation was also observed by other authors [19,31],and it indicates that adsorption must occur with hy-droxyl groups located either on the outer surface ornear the pore mouth.

Even though the concentrations of acid sites inmedium pore SAPO-11, SAPO-31 and SAPO-41,measured through ammonia TPD-MS, are very close,there is some inconsistency in the results obtainedwith pyridine. This could be attributed to the smallerpore openings of SAPO-31, which may hamper theaccess of pyridine molecules.

Fig. 3. FT-IR spectra of SAPO molecular sieves used in this study: (a) SAPO-5, (b) SAPO-11, (c) SAPO-31, (d) SAPO-34 and (e) SAPO-41.

As a general result, we can observe a good corre-spondence between the acidity values obtained withthese two probes (pyridine and ammonia), except forSAPO-34 in which its lower pore diameter imposessterical restrictions to pyridine diffusion inside thepore system, and NH3 seems to detect more sites thanpyridine.

The IR spectra in pyridine (1575–1425 cm−1) aswell as the hydroxyl stretching (4000–3500 cm−1)region are often used to investigate the nature ofBrönsted and Lewis acid sites present in various zeo-lites and for metal substituted molecular sieves [32].The IR spectra of framework vibrations are shownin Fig. 3. These spectra are quite similar, which isconsistent with the results in the literature [20]. Fig.3 also shows the hydroxyl IR spectra of SAPOs inwhich five different bands of free hydroxyl groups


Fig. 4. IR spectra after pyridine adsorption at room temperature and following desorption at 100, 200 and 300◦C, on SAPO-5 (left) andSAPO-11 (right) solids.

are observed with maximum absorbance at ca. 3828,3735, 3666, 3600 and 3534 cm−1. These bands maybe attributed to AlOH groups, SiOH groups fromextracrystalline silica impurities [33], POH groups[34,35] and two types of SiOHAl (bridging hydroxyl)groups [36,37], respectively. The last two peaks arisefrom the substitution of silicon for phosphorus in theSAPO lattice, SAPO-5 being the solid where thesepeaks have the highest intensity. The distribution ofOH groups is very similar in SAPO-11, SAPO-31 andSAPO-41 (where two bands can be observed, one at3735 cm−1 attributed to silicon-hydroxyl groups andanother at 3666 cm−1 due to phosphorus-hydroxylgroups), while SAPO-5 presents multiple vibrationsof hydroxyl groups of great intensity. These resultsclearly indicate important differences in the type ofacid sites found in catalysts.

Fig. 4 shows the infrared spectra after the adsorptionand desorption of pyridine between 100 and 300◦C,for SAPO-5 and SAPO-11 catalysts. In all cases, weobserve adsorption peaks at ca. 1632 and 1540 cm−1,which correspond to Brönsted acid sites, and peaks atca. 1614 and 1450 cm−1, due to Lewis centers [38].The band at ca. 1490 cm−1 is due to Lewis and Brön-sted acid sites. The adsorption peak at 1540 cm−1 cor-responds to the Brönsted acidic center induced by Si+4

substitution for P+5 in the aluminophosphate frame-work [39–41]. The overall intensity of these bands ishigher for SAPO-5 than for SAPO-11, -31, -41 and-34. In the case of medium pore SAPOs, these bandspractically disappear at 300◦C indicating their loweracidity, while for SAPO-5, the intensity at 300◦C isstill important. The SAPO-34 sample shows an ab-sence of bands; this does not mean that SAPO-34 doesnot contain Brönsted and/or Lewis acid centers. It sim-ply suggests that, because of the smaller pore size ofSAPO-34, pyridine molecules are not able to reach theacid sites situated inside the pores.

From the analysis of the TPD-MS of ammonia andIR chemisorbed pyridine, it could be concluded thatthese catalysts have Brönsted acid sites and that theacid strength of SAPO-11, SAPO-31 and SAPO-41 isweaker than that of SAPO-5 and SAPO-34.

3.3. Catalytic activity

Methanol conversion was used in the present studyto evaluate catalytic properties of SAPO solids withdifferent structures and acidities. It is well known thatmethanol conversion can proceed on molecular sieveseither as a simple dehydration, yielding dimethyl ether

https://www.researchgate.net/publication/250741943_The_acidic_properties_of_SAPO-37_compared_to_faujasites_and_SAPO-5?el=1_x_8&enrichId=rgreq-b0f87a4ec07ad80de1d6bf1e324df47b-XXX&enrichSource=Y292ZXJQYWdlOzIyMjg2OTg2OTtBUzo5OTA5MTk3MTMxMzY2OEAxNDAwNjM2Nzc1NzE0


Fig. 5. Conversion of methanol with SAPOs at 400◦C.

(DME) and water as the only products, or as a processof deep dehydration, producing hydrocarbons [42].

Fig. 5 shows the catalytic performance of thesesolids with methanol, under the same reaction condi-tions. SAPO-34 and SAPO-5 are, initially, the mostactive solids in this reaction, whereas SAPO-11, -31and -41 show lower activities, which corresponds tothe acidity sequence shown previously by ammoniaTPD-MS. However, the activity falls very quickly inthe case of the most active solids.

A certain product distribution is characteristic foreach catalyst. In Table 4, we can observe initial selec-tivities to different products (t = 0 h), and selectivitieswhen catalysts are deactivated (t = 10 h). Initially,SAPO-5 and SAPO-34 show high selectivities to

Table 4Product distribution (wt.%) obtained initially and after 10 h with methanol

Catalyst Conversion Dimethyl ether Ethylene Propylene C4 C5+

t = 0 t = 10 t = 0 t = 10 t = 0 t = 10 t = 0 t = 10 t = 0 t = 10 t = 0 t = 10

SAPO-5 93.07 6.72 24.51 6.33 5.87 0.12 14.30 0.27 38.95 – 9.44 –SAPO-11 72.04 28.19 69.19 28.02 0.39 0.06 1.23 0.11 0.76 – 0.47 –SAPO-31 65.66 41.48 65.01 41.35 0.14 – 0.35 0.15 0.05 – 0.11 –SAPO-34 95.89 16.48 1.98 16.48 24.33 – 46.58 – 18.19 – 4.81 –SAPO-41 57.40 11.52 46.82 11.39 2.10 – 3.99 0.13 2.85 – 1.64 –

Fig. 6. IR spectra of deactivated catalysts: (a) SAPO-34, (b)SAPO-5 and (c) SAPO-11.

hydrocarbons; SAPO-34 to lower olefins in the se-quence C3 > C2 > C4, aromatics and branched isomersare not detected, whereas on SAPO-5, the selectivitytowards C4+ hydrocarbons (hydrocarbons with four ormore carbons) is very important, with the presence ofbranched hydrocarbons and a very small amount ofaromatics. The selectivity shown by SAPO-34 may beexplained by its pore size, which is smaller than thekinetic diameter of molecules such as aromatics andbranched isomers [43,44] and assumes a low concen-tration of acid sites on the external surface in relationto that on the internal surface (this may be assumedby the very low ratio micropore area to BET area andthe low pyridine adsorption values obtained).

The solids with medium-pore structures, SAPO-11,-31 and -41, show low activity towards the formation


Fig. 7. Total ion current MS profile from TPD of deactivatedSAPO-5 (lower) and SAPO-34 (upper).

of hydrocarbons, DME being the main productformed. It is natural to assume that similar to thezeolite catalysts, both weak and strong acid sites inSAPO will facilitate the simple dehydration reactionbut only strong acid sites will facilitate the formationof hydrocarbons. This correlation between strong acidsites and the formation of hydrocarbons is what wereally observe with SAPO-5 and SAPO-34, whereasthe solids with the highest proportion of weak acidsites exhibit the highest DME selectivity.

3.4. Characterization of coke

In all cases, DME is, practically, the only prod-uct obtained after several hours. Deposition of cokewas significant in the most active catalysts (SAPO-5,SAPO-34) and less important in the other solids. Thiscoke was characterized by techniques such as FT-IR,MAS-NMR and high resolution mass spectrometry.

FT-IR spectra of the deactivated solids (Fig. 6) showthe presence of several bands centered at ca. 2928 and2870 cm−1 which correspond to aliphatic C–H stretch-ing vibrations and at ca. 1381, 1462 and 1585 cm−1

attributed to aliphatic C–H bending vibrations. Thesebands indicate the nature of the coke formed insidethe pores of the catalysts, it being more intense forSAPO-34 (catalysts whose deactivation is very fast)and less intense for SAPO-11, -31 and -41 (practically,they do not appear). The presence of negative bandsin the hydroxyl stretching region indicates that cokeinteracts with Brönsted centers; these negative bandsare more intense for SAPO-34 and less intense forSAPO-11, -31 and -41.

Fig. 7 shows the MS total ion current obtainedfrom deactivated solids (SAPO-5 and SAPO-34)heated from 150 to 600◦C at 2◦C s−1 under vac-uum (10−6 mbar) at the source housing of the massspectrometer. In this case, we can observe the cokedesorption with temperature that takes place at highertemperature in the case of SAPO-34, indicatingthe diffusional restrictions that coke molecules findthrough the pores of this catalyst to abandon its mi-croporous system. EI mass spectra obtained from TICare shown in Fig. 8. In both cases, we can observebreakdown peaks from large polyunsaturated hydro-carbon chains, whose elemental composition (accord-ing to high resolution mass spectra) are C4H7 (m/z55). C4H9 (m/z 57), C5H9 (m/z 69), C5H11 (m/z 71),C6H9 (m/z 81), C6H13 (m/z 83)...; on increasing themolecular weight of the breakdown ion, the intensityof polyunsaturated peaks increases and indicates thehigh degree of unsaturation of these chains (i.e.,m/z81 corresponds to a fragment with two unsaturations).

1H MAS NMR spectra of deactivated SAPO-5,SAPO-34 and SAPO-31 are shown in Fig. 9. WithSAPO-34 and SAPO-5, several resonances with themaximum at chemical shifts of ca. 0.99–1.44 ppmare observed, which can be attributed to hydrocarbonchains which may contain some double bounds. Sincerecently, we practically do not observe the presenceof aromatic signals. The presence of resonances withSAPO-31 are negligible according to its very lowcontent of coke as we have shown previously withmedium pore SAPOs. Fig. 9 also shows13C MASNMR of deactivated SAPOs, they being the mostintense signals due to hydrocarbon molecules withseveral double bounds (6–35 ppm).


Fig. 8. EI mass spectra obtained from TPD of deactivated SAPO-5 (lower) and SAPO-34 (upper).

In order to explain the deactivation of SAPO-34,it is important to take into account the chabazite-likestructure, with pores of 0.44 nm interconnected bysupercages which are bigger than these pores [8].Oligomers formed inside the pores can migrate fromthe small pores to big cages and interact with strongacid sites; these oligomers can be adsorbed stronglyon such acid sites and poison them (alkenes andoligomers can interact very strongly preferably withBrönsted acid sites [45]), decreasing the concentra-tion of strong acid sites as well as the conversion tolight alkenes. Heavier oligomers can be formed fromlighter oligomers inside the supercages [43,46,47],and as the pore size of SAPO-34 is smaller than thekinetic diameter of heavy oligomers, these cannotleave the cages, leading to pore blockage. The ulti-mate conversion is not zero due to the fact that theequilibration reaction between methanol and dimetylether can take place even on the weak acid sites foundon the external surface.

The unidimensional pore structure of SAPO-5,with non-interconnecting uniform channels with a0.74 nm diameter, explains how the formation ofheavy oligomers with multibranched chains over

strong acid sites can cause blockage of the pore sys-tem. This in turn prevents the reactant moleculesfrom having access to the acid sites since this block-age is dominant due to their lack of interconnection[48]. The presence of weak acid sites on mediumpore SAPOs explains the low deactivation observedin these solids, in which precursor coke compounds(olefins) are formed in minimal quantities.

4. Conclusions

The usefulness of SAPOs in obtaining hydrocarbonsfrom methanol is highly influenced by the acidity andpore size of the molecular sieve. Diffusional resistancewithin the micropores of the SAPO-34 molecular sievelimits the product spectrum to light olefins. On theother hand, the presence of bigger pores on SAPO-5facilitates the formation of higher alkenes with thepresence of branched isomers.

The presence of acid sites with more or less activ-ity is another important parameter in the selectivityof the process. Solids with a great proportion ofBrönsted silicon-hydroxyl and phosphorus-hydroxyl


Fig. 9. 1H-MAS NMR spectra (right) and13C-MAS-NMR spectra (left) of deactivated SAPO-34 (a), SAPO-5 (b) and SAPO-31 (c).

groups (SAPO-11, SAPO-31 and SAPO-41) are lessactive in hydrocarbon production than those withaluminum-hydroxyl groups (SAPO-5 and SAPO-34).

The formation of coke is derived from the oligomer-ization process of olefins formed inside the pore sys-tem; the formation of these oligomers takes place onstrong acid sites which interact strongly with them andmay cause the blockage of the pore system.

Acknowledgements

Authors acknowledge the support of the MassSpectrometry Service of the Cordoba University. Thisresearch was subsidized by the DGESIC (ProjectPB97/0446), Ministerio de Educación y Cultura, andby the Consejerıa de Educacióm y Ciencia (Junta deAndalucıa, Spain). They also thank Prof. M. Sullivanfor linguistic revision of the manuscript.

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